1. Field of the Invention
The present invention relates to communications systems. More specifically, the present invention relates to systems and methods for protecting Global Positioning System (GPS) receivers from jamming and interference.
2. Description of the Related Art
The Global Positioning System includes a constellation of satellites that continuously transmit signals relating to the positions thereof and time. When received by a GPS receiver, the signals are processed to provide time of arrival information. The time of arrival of the pulses from each satellite are then used to calculate the location or position of the receiver. Inasmuch as the GPS system is effective for accurate position location, in certain e.g., military, applications, there is a concomitant motivation to jam or interfere with the GPS system. The interfering signal from the jammer is designed to introduce unacceptable error in the GPS position estimate.
Accordingly, countermeasures have been developed to compensate for jamming interference. One widely used countermeasure involves adaptive array processing. These systems generally include an array of antenna elements and a system for processing outputs thereof.
Unfortunately, adaptive array processing systems generally offer a limited capability over a broad bandwidth and, in addition, cause errors in GPS precision when used for interference rejection. That is, a typical GPS receiver operates at approximately 1.5 GHz with a bandwidth of approximately 20 MHz. This relatively wide bandwidth makes it difficult to cancel jamming with conventional adaptive array processing techniques. This is due to the fact that conventional adaptive array processing techniques operate on the basis of a presumption of a very narrow bandwidth e.g., 1 Hz. Therefore, when these techniques are applied to systems with finite bandwidth, they can suffer a significant performance degradation.
These problems are exacerbated when the particularly novel and advantageous technique of band partitioning is employed such as that set forth in U.S. Pat. No. 4,821,037 issued Apr. 11, 1989, to Miller et al, and entitled WIDEBAND ADAPTIVE ARRAY USING THE CHIRP TRANSFORM, the teachings of which are incorporated herein by reference. Band partitioning involves breaking up the GPS frequency band into a number of narrower bandwidth subbands, adaptive array processing the partitioned subbands and reassembly of the subbands to reconstruct the GPS frequency band. This approach significantly improves performance by performing adaptive array processing on many narrow band signals rather than on one wideband signal.
Unfortunately, band partitioning yields a different set of adaptive nulling weights for each frequency band. While effective over the limited subbands, nulling on reassembly of the partitioned bands is problematic. A particularly troublesome aspect of the problem has to do with the fact that when the spectrum is reassembled, a time delay distortion and bias is created which is dependent on the the adaptive array weights, which are in turn dependent on the unknown location of the jammer(s). This is particular problematic inasmuch as time delay is used in the normal course of GPS processing to estimate or determine the position of a receiver. If the time delay were fixed across the constellation, it would be relatively easy to take into account in the process of computing the time delay associated with valid GPS signals. Unfortunately, the delay is a function of the angle of arrival of the satellite signal (i.e., the time delay experienced by the satellite signal is a function of the position of the satellite, and therefore differs among the different satellites). As the location of the jammer is unknown, and furthermore because the jammer spectra tends to vary from antenna element to antenna element due to jammer multipath and small differences between the antenna channels, computation of the time delay bias due to interference from a jammer remains a daunting task in an adaptive array GPS processing system.
Hence, there is a need in the art for a system or method for preserving the precision measurement capability of a GPS receiver when an adaptive processing system or algorithm is used to compensate for jamming interference.